The invention relates to the stimulation of the nervous system of humans by an electric field applied externally to the body. A neurological effect of external electric fields has been mentioned by Norbert Wiener [1], in discussing the bunching of brain waves through nonlinear interactions. The electric field was arranged to provide "a direct electrical driving of the brain" [1]. Wiener describes the field as set up by a 10 Hz alternating voltage of 400 V applied in a room between ceiling and ground.
Brennan [2] describes an apparatus for alleviating disruptions in circadian rythms of a mammal, in which an alternating electric field is applied across the head of the subject. The voltage applied to the electrodes is specified as at least 100 V, and the peak-to-peak value of the electric field as at least 590 V/m in free air before deploying the electrodes across the head of the subject. The frequency of the alternating electric field is in the range from 5 to 40 Hz. Brennan states that the method is aimed at subjecting at least part of the subject's brain to an alternating electric field. It should be noted that electric polarization of the head causes the field strength in the narrow space between electrode and skin to be about a factor h/2d larger than the free-air field strength, h being the distance between the electrodes and d the spacing between electrode and skin. For h=17 cm and d=5 mm the factor comes to 17, so that with the specified free-air field of at least 590 V/m the field in the gap between electrode and skin is at least 10 KV/m peak to peak.
A device involving a field electrode as well as a contact electrode is the "Graham Potentializer" mentioned in Ref. [3]. This relaxation device uses motion, light and sound as well as an alternating electric field applied predominantly to the head. The contact electrode is a metal bar in Ohmic contact with the bare feet of the subject; the field electrode is a hemispherical metal headpiece placed several inches from the subject's head. According to the brief description in [3], a signal less than 2 V at a frequency of 125 Hz is applied between the field electrode and the contact electrode. In this configuration the contact electrode supplies to the body the current for charging the capacitor formed by the field electrode and the apposing skin area. The resulting electric field stands predominantly in the space between the head piece and the scalp.
In the three external field methods mentioned, viz. Wiener [1], Brennan [2], and Graham [3], the electric field is applied to the head, and the brain is thereby exposed to polarization currents. These currents run through the brain in a broad swath, with a distribution influenced by nonuniformities of tissue conductivity and permittivity. The scale of the current density can be conveniently expressed by the maximum value, over the skin of the head, of its component perpendicular to the local skin. This scale is easily calculated for sinusoidal fields as the product of radian frequency, vacuum permittivity, and maximum amplitude of the external field on the head. Using Brennan's [2] lowest frequency of 5 Hz, his miniumum required free-air field strength of 590 V/m, and the factor 17 estimated above to account for the polarization of the head by the applied field, the scale of the polarization current density in the brain comes to about 280 pA/cm.sup.2. Although such a current density would generally be considered very small in electrophysiology, a repetitive application at certain critical frequencies and along certain paths in the brain may perhaps cause kindling in individuals predisposed to epilepsy, and it is therefore deemed too large for use by the general public. Conservatively, we consider polarization current densities in the brain with a scale in excess of 70 fA/cm.sup.2 to be substantial.
It is the object of the present invention to obtain a method and apparatus for manipulating the nervous system by externally applied electric fields without causing substantial polarization current densities in the brain.
The use of electric fields raises concerns about possible health effects. Such concerns have been widely discussed in the media in regard to electric power lines and electric apparatus [4]. Answering the pertinent questions by objective research will take time, but meanwhile governments have been setting guidelines for safe limits on field strengths. At present, the strictest limits of this sort are the Swedish MPRII guidelines. Magnetic fields are of no concern here, because the currents involved are so small. However, the electric field strengthy must be considered, since even at low voltages strong electric fields can result from electrodes placed close to the skin. For fields of extremely low frequency the MPRII guidelines limit the field strength to 25 V/m in the frequency range from 5 Hz to 2 KHz. In the Brennan patent [2] the minimum field strength of 590 V/m violates these guidelines by a factor 23; when the polarization effects are accounted for, the factor is about 400.
It is a further object of the present invention to manipulate the nervous system by external electric fields that are in compliance with the MPRII guidelines.
Brennan [2] stipulates voltages of at least 100 V, and as high as 600 V for his preferred embodiment. Generation of such voltages requires a voltage multiplier stage, if practical battery operation is desired. This increases the current drain and the size of the generator. The large voltages also raise safety concerns.
It is yet a further object of the present invention to manipulate the nervous system by external electric fields, using low voltages that are generated by small and safe battery-powered devices with low current consumption.
The nervous system generally habituates to repetitive stimuli. It is still a further object of the present invention to manipulate the nervous system by externally applied electric fields in such a manner as to thwart habituation.